Cell adhesion plays an important role in sustaining life of multicellular organisms. Cell adhesions of multicellular organisms are classified into cell-extracellular matrix adhesion and cell-cell adhesion. It has been elucidated that the cell-extracellular matrix adhesion is mediated by integrins and the cell-cell adhesion is mediated by cadherins, claudins, nectins, etc. In addition, it is becoming clear that these adhesion molecules not only play a role in cell adhesion, but also are directly involved in signal transduction into cells.
Cell-extracellular matrix adhesions are consisted of transmembrane adhesion proteins such as integrins. It is reported that integrin forms heterodimer of α and β chains. There are at least 24 types of integrin molecules, which depend on the type of combination of α chain and β chain. Each type of integrin binds to a specific extracellular matrix molecule. Transmembrane adhesion proteins including integrins are involved in not only cell-extracellular matrix adhesions but also intracellular signal transductions from extracellular matrix and regulation of proliferation, mobility, apoptosis and differentiation (F. G Giancotti, et al., Science, 285, 1028-1032, 1999).
Many proteins are known as extracellular matrix molecules which are classified into collagens (such as types I to XIX), non-collagenous glycoproteins (such as osteopontin, vitronectin, fibronectin, von Willebrand Factor, laminin, tenascin, fibrinogen, thrombospondin), elastins and proteoglycans (such as heparan sulfate proteoglycan). It is appeared that these extracellular matrix molecules (ligands) bind to corresponding integrins and activate intracellular signal transduction pathways to regulate cytoskeltal organization, mobility, proliferation, differentiation and the like. That is, integrins which bind to ligands cooperate with cell-surface receptor-type tyrosine kinase to regulate these signal activating pathways by transmitting specific signals depending on the type of ligand. It is appeared that RGD (Arginine-Glycine-Asparagine acid) sequence is commonly observed in cell adhesion region of many extracellular matrix proteins. Therefore, since the RGD sequence of extracellular matrix proteins binds to integrins to exhibit various functions, the RGD sequence can be a medicinal target, and many small-molecular compounds and synthetic peptides have been provided.
As integrins which bind to the RGD sequence, α3β1 integrin, α5β1 integrin, α8β1 integrin, αvβ1 integrin, αvβ3 integrin, αvβ3 integrin, αvβ6 integrin, and αvβ8 integrin are present. Mechanisms of integrin mediated signal transduction has been studied mainly with interaction between α5β1 integrin and its specific ligand fibronectin, and it is reported that α5β1 integrin regulates not only cell adhesion and cell mobility but also cell differentiation and cell mortality (S. M. Frisch et al., Curr. Opin. Cell Biol., 9, 701-706, 1997). However, each integrin mediated signal differs depending on the type of the ligand. For example, fibronectin-bound endothelial cells show proliferation by stimulation of growth factor, but when similar cells bind to laminin-1, the growth is inhibited. Further, the signal transmitted from laminin-10/11 via α3β1 integrin is different from the signal transmitted from fibronectin via α5β1 integrin, and significantly enhances a mobility of tumor cells (J. Gu et al., J. Biol. Chem., 276, 27090-27097, 2001) and significantly avoids apoptosis by blood starvation (J. Gu et al., J. Biol. Chem., 277, 19922-19928, 2002). Among the integrins which bind to the RGD sequence, high expression of αv integrins has been observed in the osteoclast and neovascular, and the αv integrins have been expected as target molecules for a therapeutic medicine for osteoporosis and cancer. It has been indicated that α5β1 integrin are highly expressed on tumor cells and involved in malignancy of tumor cells. Based on these findings, anti-α5β3 integrin antibody (Volocimab), anti-α4 integrin antibody (Natalizumab), and anti-αvβ3 integrin antibody (Vitaxin) have been developed as antagonistic antibody medicines which inhibit binding of extracellular matrix protein to integrin.
Meanwhile, some extracellular matrix proteins such as collagen, osteopontin (OPN), vitronectin, fibronectin, von Willebrand Factor, laminin, tenascin, fibrinogen and thrombospondin have been known to include RGD sequence. Also, some virus and some bacterium have RGD sequence which is concerned in adhesion to cells. OPN, which is contained rich in bone and includes RGD sequence, is an acidic glycoprotein with binding properties to calcium which is contained rich in bone. It is reported that OPN plays an important role in cell adhesion, cell migration, tumor formation, immune response, complement mediated cellular lysis, etc. Analyses using OPN knockout mice and anti-OPN neutralizing antibodies indicate that OPN relates to hepatitis, autoimmune disease such as rheumatoid arthritis and metastasis of cancer. Therefore, it is expected that inhibition of binding of extracellular matrix proteins to cells via RGD may be used for a treatment of osteoporosis or cancer. Thus, in addition to the above mentioned antagonistic medicines targeted to integrins, antagonistic medicines targeted to the extracellular matrix proteins have been developed.